Seismic waves generated for the imaging of geological layers have been used for more than 50 years. The most widely used waves are reflected waves and more precisely reflected compressional waves. During some land seismic prospection operations, vibrator equipment (also known as a “source”) generates a seismic motion that propagates in particular in the form of a wave that is reflected on interfaces of geological layers. Alternatively other sources of elastic waves can be used. These waves are received by, for example, geophones, hydrophones or other types of receivers, which convert the received elastic waves into an electrical signal which can be recorded and analyzed. For example, analysis of the arrival times and amplitudes of these waves makes it possible to construct a representation of the geological layers from which the waves are reflected.
FIG. 1 depicts schematically a land seismic exploration system (system) 70 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. At least one purpose of system 70 is to determine the absence, or presence of hydrocarbon deposits 44, or at least the probability of the absence or presence of hydrocarbon deposits 44, which are shown in FIG. 1 as being located in first sediment layer 16. System 70 comprises a source consisting of a vibrator 71, located on first vehicle/truck 73a, operable to generate a seismic signal (transmitted waves), a plurality of receivers 72 (e.g., geophones) for receiving seismic signals and converting them into electrical signals, and seismic data acquisition system 200 (that can be located in, for example, vehicle/truck 73b) for recording the electrical signals generated by receivers 72. Source 71, receivers 72, and data acquisition system 200, can be positioned on the surface of ground 75, and all interconnected by one or more cables 12. FIG. 1 further depicts a single vibrator 71 as the source of transmitted elastic waves, but it should be understood by those skilled in the art that the source can actually be composed of one or more vibrators 71. Furthermore, vehicle 73b can communicate with vehicle 73a via antenna 240a, 240b, respectively, wirelessly. Antenna 240c can facilitate communications between receivers 72 and second vehicle 73b and/or first vehicle 73a. 
Vibrator 71 is operated during acquisition so as to generate a seismic signal. This signal propagates on the surface of ground 75, in the form of surface waves 74, and in the subsoil, in the form of body waves 76 that generate reflected waves 78 when they reach an interface 77 between two geological layers, first and second layers, 16 and 18, respectively. Each receiver 72 receives both surface wave 74 and reflected wave 78 and converts them into an electrical signal in which are superimposed the component corresponding to reflected wave 78 and the component that corresponds to surface wave 74, the latter of which is usually considered undesirable and should be filtered out as much as is practically possible.
The vibrator 71 applies a vertical force to earth 75 and unlike impulsive sources, spreads the energy over time. This is accomplished by providing an input sweep signal to the vibrator 71. The typical sweep signal of choice in the seismic exploration industry in recent times has been the sine-wave up-sweep signal, s(t), expressed as a function of time as follows:s(t)=A0(t)cos(ϕ(t))  (1),where A0 is a constant and ϕ(t) is a phase with an instantaneous phase value ϕ(t) expressed as:ϕ(t)=2π∫0tf(τ)dτ+ϕ0  (2)and an instantaneous frequency, f(t):f(t)=fmin+∫0tSr(τ)dτ  (3)where fmin is the minimum frequency, Sr is the positive sweep rate and ϕ0 is the initial phase. As those of skill in the art can appreciate, sine-wave signals have low autocorrelation side-lobes, and a meaningful instantaneous phase expression that was well suited for older generations of vibrator electronics performing phase-locking. The up-sweep signal, i.e., a signal sweeping from low-to-high frequencies, is the standard chosen for the behavior of cross correlated harmonic distortion. With a positive sweep rate, however, undesirable energy is produced, i.e., the correlated harmonic distortion, which leaks towards the negative times following correlation.
An alternative to sine-wave upsweep signals as inputs to vibrators 71 are pseudo-random sequence sweep signals, which are spectrally shaped to meet the bandwidth and amplitude requirements for seismic investigation. Pseudo-random sequences are known to have higher autocorrelation sidelobes than sine-wave sweeps and, for such signals, instantaneous frequency has little meaning as this quantity assumes that a given frequency is concentrated around a single time instant
Regardless of whether sine-wave sweeps or pseudo-random sweeps are used in seismic acquisitions, the resulting acquired seismic data is processed to remove noise. For example, various phenomena, both natural and man-made, may introduce noise bursts during land seismic acquisitions. For example, vehicles traveling on roads nearby the area in which seismic acquisition is being performed can generate such noise bursts. Impulsive noise bursts acquired during seismic recording will undergo correlation or convolution processes which will spread their energy over time in the process.
One technique used to mitigate noise-bursts is the so-called diversity stack technique. The diversity stack technique requires acquisition of the same signal, several times, at the same location, and then combining the results as a weighted sum, e.g., as described in U.S. Pat. No. 3,398,396, the disclosure of which is incorporated here by reference. While the diversity stack is efficient at removing noise, there is a requirement for a listen time between each sequence that makes it more expensive than a single longer sweep.
Accordingly, it would be desirable to provide methods and systems for the generation of seismic signals, or more specifically sweep signals, that avoid the problems of previous solutions, e.g., having to repeat measurements to employ the diversity stack technique.